RecurrentLayer bugfix: params still need backprop

src/caffe/layers/recurrent_layer.cpp

@@ -208,8 +208,12 @@ template <typename Dtype>
 void RecurrentLayer<Dtype>::Backward_cpu(const vector<Blob<Dtype>*>& top,
     const vector<bool>& propagate_down, const vector<Blob<Dtype>*>& bottom) {
   CHECK(!propagate_down[1]) << "Cannot backpropagate to sequence indicators.";
-  if (!propagate_down[0] && !propagate_down[2]) { return; }
 
+  // TODO: skip backpropagation to inputs and parameters inside the unrolled
+  // net according to propagate_down[0] and propagate_down[2]. For now just
+  // backprop to inputs and parameters unconditionally, as either the inputs or
+  // the parameters do need backward (or Net would have set
+  // layer_needs_backward_[i] == false for this layer).
   unrolled_net_->Backward();
 }